workqueue: add printf attribute to __alloc_workqueue()

Fix a compiler warning with W=1:
kernel/workqueue.c: error:
function ‘__alloc_workqueue’ might be a candidate for ‘gnu_printf’
format attribute[-Werror=suggest-attribute=format]
 5657 |  name_len = vsnprintf(wq->name, sizeof(wq->name), fmt, args);
      |  ^~~~~~~~

Fixes: 9b59a85a ("workqueue: Don't call va_start / va_end twice")
Signed-off-by: Su Hui <suhui@nfschina.com>
Signed-off-by: Tejun Heo <tj@kernel.org>